In computer graphics and geography, the illumination angle of a surface with a light source (such as the Earth's surface and the sun) is the angle between the inward surface normal and the direction of light. It can also be equivalently described as the angle between the tangent plane of the surface and another plane at right angles to the light rays. This means that the illumination angle of a certain point of the Earth's surface is zero if the Sun is precisely overhead and that it is 90 degrees at sunset and at sunrise.